Thermal management device and method of making such a device

ABSTRACT

A thermal management device ( 13 ) comprising anisotropic carbon ( 10 ) encapsulated in an encapsulating material ( 12 ) that improves the strength of the carbon. The encapsulating material may b polyimide or epoxy resin or acrylic or polyurethane or polyester ( 12 ) or any other suitable polymer.

[0001] The present invention relates to a thermal management dew-ice formanaging the dissipation of heat in, for example, electronic equipmentand a method of making such a device. In particular, the inventionrelates to a thermal management device that has electrical feed throughcapability and can act as a direct interface to active elements.

[0002] Electronic and electrical devices arc the sources of both powerand heat. As is well known, in order to provide reliable operation ofsuch devices, it is necessary to maintain stable operating conditionsand temperatures. Hence, efficient methods for heat management anddissipation are essential. Typically this is done by providing thermalmanagement devices that are arranged adjacent and in contact with theelectronic device or circuit board. Heat generated in the circuit istransferred to and dissipated in the thermal management device. Foroptimum efficiency, it is desirable that thermal management structureshave the highest possible thermal conductivity, efficient externalconnectivity and appropriate mechanical strength.

[0003] To achieve these objectives in thermally demanding applications,some known devices encapsulate high thermal conductivity materials intocomposite structures. However, these devices often achieve only limitedperformance, with significant conductivity losses, typically 40%, andincreases in mass and bulk. Examples of such structures are described inEP 0,147,014, EP 0,428,458, U.S. Pat. No. 5,296,310, U.S. Pat. No.4,791,248 and EP 0,231.823. The best thermal management systemsavailable at present have conductivities that typically do not exceed1,000W/mK.

[0004] Current technologies do not provide thermal management that issufficient in many applications whilst at the same time providingefficient electrical interconnection between layers or sides of circuitboards. A further problem is that the mass and volume of known thermalmanagement systems are relatively large. This affects the overall sizeof electronic systems in which such devices are incorporated. In thisday and age when the general drive of the electronics industry istowards miniaturisation, this is highly disadvantageous.

[0005] Thermal management systems are often used as substrates forsupports for hybrid electronic circuits. In one known arrangement,beryllia is used as a beat sink. This has a thermal conductivity ofaround 280W/mK at room temperature. On top of this is a layer ofdielectric on which gold contacts are subsequently formed, thereby toenable connection to other electrical circuits. A disadvantage of thisarrangement is that beryllia is a hazardous material, in fact it iscarcinogenic, and is generally difficult to process. In addition, thedielectric tends to be thick thereby making the overall structure bulky.Furthermore, partly because of the use of gold as a contact material,the overall structure is expensive to manufacture.

[0006] An object of the present invention is to provide a thermalmanagement system that has a high thermal conductivity but a low massand volume.

[0007] According to a first aspect of the present invention there isprovided a thermal management device comprising anisotropic carbonencapsulated in an encapsulating material that is applied directly tothe carbon and is able to improve the rigidity of the carbon, preferablywherein the encapsulating material is polyimide or epoxy resin oracrylic or polyurethane or polyester or any other suitable polymer.

[0008] Preferably, the anisotropic carbon has mosaic or full ordering.

[0009] Preferably, the anisotropic carbon is thermalised pyrolyticgraphite that has mosaic or full ordering. The thermalised pyrolyticgraphite may have an in plane thermal conductivity of 1550-1850W/mK ataround room temperature. Typically, the thermalised pyrolitic graphitehas a low value of tensile strength in the orthogonal direction.

[0010] The anisotropic carbon may alternatively be pyrolytic graphite.The pyrolytic graphite may be in an “as deposited” or partially orderedform. The conductivity of the pyrolytic graphite may be in the range of300-420W/mK in one plane. The tensile strength of the plate may be 1.5Ksi in the orthogonal plane.

[0011] Preferably, the anisotropic carbon is a plate. Preferably thecarbon plate has a thickness in the range 100-500 μm. The carbon platemay have a thickness in the range of 200-250 μm or 250-300 μm or 300-350μm or 350-400 μm or 400-450 μm or 450-500 μm.

[0012] Preferably the material encapsulating the carbon has a lowthermal expansion coefficient and high degradation temperature, such asa polyimide, for example PI 2734 provided by DuPont (trade mark), wherethe thermal expansion coefficient is around 13 ppm/C and the degradationtemperature is around 500C.

[0013] The coating layer may have a thickness in the range from a fewmicrons to many tens of microns. Multiple layers of coating may beformed on the carbon in order to build up a desired thickness.

[0014] A matrix of fine holes, preferably 200 μm diameter, may be formedthrough the carbon plate, prior to encapsulation. These holes are filledduring encapsulation of the plate. An advantage of this is that itreduces the possibility of internal delamination.

[0015] According to a second aspect of the present invention there isprovided an electrical system comprising a thermal management device inwhich the first aspect of the invention is embodied, on a surface ofwhich electrical contacts and/or devices are provided.

[0016] The devices may be deposited directly on the surface or may beglued using, for example, a thin layer of liquid glue. Preferably, thedevices are encapsulated in polyimide or epoxy resin or acrylic orpolyurethane or polyester or any other suitable polymer.

[0017] Preferably, a plurality of layers of electrical components areprovided, each spaced apart by layers of polyimide. Typically, theelectrical contacts are made of thin film metal, for example aluminium.

[0018] According to a third aspect of the present invention there isprovided a method of fabricating a thermal management device comprising:

[0019] applying a coat of encapsulating material, preferably polyimideor epoxy resin or acrylic or polyurethane or polyester or any othersuitable polymer directly to a clean carbon surface, the encapsulatingmaterial being such as to improve the rigidity of the carbon; andrepeating the foregoing steps until the carbon is encapsulated.

[0020] The method may additionally involve curing the encapsulatingmaterial.

[0021] Preferably, the step of applying involves brushing, rolling,dipping, spraying, spinning, stamping or screen-printing. Preferably,for polyimide, which consists of a single-component, the step ofapplying the coating involves brushing the polyimide or applying itusing a roller. For solid phase application a cast can be used. Thisrequires a pre-polymerised foil of the encapsulating material to beapplied directly on to the clean surface. This can be useful when simplethermal management devices are required with no internal holes.Preferably, the carbon and cast are compressed within a vacuum and athigh temperature.

[0022] Preferably, the step of applying involves applying multiplelayers of encapsulating material, such as polyimide or epoxy resin oracrylic or polyurethane or polyester or any other suitable polymer,until a desired thickness is reached.

[0023] Preferably, the method includes cleaning a surface of the carbonthereby to produce said clean carbon surface.

[0024] Preferably, the step of cleaning involves using pumice powderunder water to remove loose materials, followed by drying. Preferably,the step of drying involves drying the carbon by baking the carbonsurface to remove moisture, for example, at 100C for one hour.

[0025] Preferably the step of cleaning includes degreasing the carbonby, for example, rinsing it with acetone.

[0026] When polyimide is used, it is preferable that the step of curinginvolves heating the carbon to 150C for, for example, 1 hour andsubsequently temperature cycling the board to 150C for 30 minutes, 250Cfor 30 minutes and finally 300C for 30 minutes.

[0027] In the case of epoxy, this can consist of a single component orelse be a double component mixture. For the single component type then atwo stage gluing can be carried out by firstly drying the glue to removethe solvent at a given temperature, (typically around 120C) and form asolid phase, and then heating it at a higher temperature typicallyaround 180° to complete the polymerisation. In the case of doublecomponent epoxy, the initial mixing of the components causes thepolymerisation process to begin, and the process may then need anythingbetween minutes and several hours for the process to be completed,depending upon the particular epoxy.

[0028] Preferably the method further comprises drilling the carbon withat least one hole prior to application of the encapsulating material.The at least one hole may be completely infilled with encapsulatingmaterial. The holes may be infilled with encapsulating material that ismixed with glass fibre spheres, each sphere typically having a diameterof 30 μm. This process may be carried out before the pure polyimidecoating, is used to encapsulate the surface of the plate, and canimprove the uniformity of coating thickness across the surface of theplate by preventing the possibility of thinning occurring around theedges of the initial holes in the plates. In either case, once theencapsulation process is completed, the said at least one hole isre-drilled, thereby to provide a through hole that is electricallyinsulated from the carbon core.

[0029] Preferably, a layer of a conducting material is applied to the atleast one hole to produce electrical connections, thereby to enableelectrical connections through the carbon. Preferably, the conductingmaterial is a metal, for example thin film aluminium. Alternatively, theedges that define the at least one hole may be coated with theencapsulating material in such a way as to maintain a passage throughthe carbon, thereby to avoid having to conduct the step of drillingthrough encapsulating material.

[0030] The method may further involve forming a matrix of fine holesthrough the carbon. These holes are of course infilled when the plate isfully encapsulated.

[0031] According to a fourth aspect of the present invention, there isprovided a method of fabricating an electrical component comprising themethod of the third aspect of the present invention and additionally thesteps of forming electrical contacts on at least one surface of thecarbon and/or depositing electrical devices thereon.

[0032] The step of depositing may involve fabricating the devicesdirectly on the surface or forming the devices or a thin filmmulti-layer circuit containing the devices separately from the carbonsurface and fixing them to that surface. Preferably, the step of fixinginvolves applying glue to the devices or the circuit or the carbonsurface and pressing the devices or circuit and the surface together atroom temperature and at low vacuum.

[0033] Preferably, the electrical contacts are applied using thin filmprocessing techniques, using, for example, aluminium.

[0034] Various devices and methods in which the present invention isembodied will now be described by way of example only and with referenceto the following drawings, of which:

[0035]FIG. 1 is a cross-section through a carbon plate;

[0036]FIG. 2 is a cross-section through a plate that has been partiallycoated with an encapsulating material, such as polyimide or epoxy resin-or acrylic or polyurethane or polyester or any other suitable polymer;

[0037]FIG. 3 is a cross-section through a plate that has been fullyencapsulated with the encapsulating material;

[0038]FIG. 4 is a cross-section through a carbon plate that has beendrilled is with holes;

[0039]FIG. 5 is similar cross-section to that of FIG. 4 except that theplate has been coated with encapsulating material;

[0040]FIG. 6 is a cross-section similar to that of FIG. 5 except thatholes are formed through the encapsulating material;

[0041]FIG. 7 is a cross-section similar to that of FIG. 6 in which theplate has been covered with metal;

[0042]FIG. 8 is a cross-section similar to that of FIG. 7 in whichinterconnection structures have been etched on both sides of the plate;

[0043]FIG. 9 is a cross section through a plate similar to that shown inFIG. 3 onto which a multi-layer electrical circuit has been directlyfabricated;

[0044]FIG. 10 is a cross section similar to that of FIG. 9, but in thiscase the multi-layer electrical circuit has been fixed to a surface ofthe carbon plate using epoxy resin;

[0045]FIG. 11 is similar to FIG. 10, except that the multi-layer circuitis mounted using epoxy on a surface of a carbon plate that has beencoated with polyimide;

[0046]FIG. 12 is a cross section through a structure that is similar tothat of FIG. 9, except a compensatory layer is included on the back-sideof the structure;

[0047]FIG. 13 is a cross section through a structure that is similar tothat of FIG. 11, except a compensatory layer is included on theback-side; and

[0048]FIG. 14 is a top view of a large carbon plate, on which is aplurality processed sites.

[0049]FIG. 1 shows a carbon plate 10. This is typically thermalisedpyrolytic graphite with mosaic or full ordering, with an in planethermal conductivity (indicated by arrow A) of 1550-1850W/mK and athermal conductivity of 825W/mK in the orthogonal direction (indicatedby arrow B), with both directions having low values of tensile strength.This material is friable, breaks easily and so is generally difficult tohandle. In addition, due to its inherent softness and layered nature anycontact with this material results in small traces of it beingtransferred to the surface it touched. This is disadvantageous inelectrical circuits where any stray shards or pieces of conductingmaterial can result in electrical shorts being formed.

[0050] The plate 10 may alternatively be pyrolytic graphite in an “asdeposited” or partially ordered form. This material is anisotropic andtypically has a thermal conductivity in the region of 300-420W/mK in oneplane (indicated generally by arrow A in FIG. 1) and 3W/mK in theorthogonal direction (indicated by arrow B in FIG. 1) with respectivetensile strengths of 14 Ks and 1.5 Ksi.

[0051] The plate 10 may have a thickness in the range 100-500 μm,preferably 200 μm, although could have any thickness suitable for agiven application.

[0052] In order to form a thermal management board that has a highthermal conductivity and is sufficiently mechanically rigid to enableelectrical components to be mounted thereon, the plate 10 is directlycoated with an encapsulating material. Suitable encapsulating materialsinclude polyimide or epoxy resin or acrylic or polyurethane or polyester12 or any other such polymer that can be applied directly to the carbonsurface and is able to improve the rigidity of the plate withoutreducing significantly its thermal conductivity. One example of asuitable polyimide is PI 2734, provided by DuPont (trade mark).

[0053] Prior to coating, a matrix of fine holes may be formed throughthe plate (not shown). The diameter of the holes is typically 200 μm.This has the advantage of reducing the possibility of internaldelamination.

[0054] In order to carry out the encapsulating process, the surface ofthe plate 10, is firstly brushed under water with pumice powder, therebyto remove any loose material. The plate 10 is dried for one hour at 100°C. and degreased with, for example, acetone. A coat of one of theencapsulating materials, for example, PI 2734 approximately 8 μm thickis then applied to one surface of the plate using a brush and the plate10 is heated for about one hour at 150° C. to partially polymerize thepolyimide. This results in one side of the plate 10 being coated withthe polyimide 12, as shown in FIG. 2.

[0055] The foregoing steps are then repeated on each side of the plate10 until it is fully encapsulated and the desired thickness of thepolyimide is reached, as shown in FIG. 3, thereby forming a thermalmanagement board 13. Generally, these steps are carried out on alternatesurfaces so that the flatness of the board can be preserved. It isimportant at this stage to ensure that all sides and edges of the plateare covered. If, however, it is necessary to contact the graphite forsome reason, small holes may be left in the polyimide, although thesewould be in-filled when the appropriate contact is made. Finally, theboard 13 is thermally cycled so that it is cured. The thermal cyclingfor a carbon plate encapsulated by PI 2734 typically involves heatingthe board 13 to 150° C. for 30 minutes, 200° C. for 30 minutes, 250° C.for 30 minutes and 300° C. for 30 minutes. If a high level of flatnessis required, then during the curing stage, the board surfaces arecompressed within a press at low vacuum.

[0056] The encapsulation process is adapted to suit the specificationand geometrical form of the required thermal management structure. Forexample, if the geometric form of the substrate includes internal holesand/or a complex perimeter with the need for all surfaces and edges tobe coated uniformly it is preferred to apply the encapsulating materialto the cleaned carbon surfaces using a brush or a roller. This allowsall surfaces and edges to be coated as required. Alternatively, thesubstrate could be coated using techniques such as dipping, spinning,spraying, stamping or screen printing. The drying, heating and optionallow vacuum pressing steps are then carried out in the same manner andsequence as previously described.

[0057] The processing steps for all the encapsulating materials areessentially the same, but as will be appreciated the temperatures usedto cause partial polymerization and curing vary. For example in the caseof epoxy resin, if type G10 FR4 is used. Once the carbon is completelyencapsulated it is heated typically to 180C for about one hour to curethe resin and thereby form the thermal management board. If required,further epoxy layers can be added by repeating the steps of applying theresin and heating the board to form an encapsulation layer of therequired thickness.

[0058] According to another encapsulation technique that uses epoxyresin, for example STYCAST (type 1266) which is a two component epoxyresin, all the resin processing steps can be carried out at roomtemperature, This minimises the possibilities of generating internalstresses or internal delamination of the substrate. The preparations ofthe substrate surfaces prior to encapsulation are carried out aspreviously described. The technique for applying the epoxy resin to thesurface of the substrate, using for example screen printing or a brushor roller, is again determined by the same considerations of substrategeometry and form.

[0059] In the case of the room temperature processing, where curingtimes can be between minutes and several hours, depending upon theproperties of the particular epoxy resin, the encapsulation processtypically has a sequence of curing procedures. An initial low vacuumenvironment encourages degassing to produce a bubble-free coating. Thisis followed by the combined application to the board of both a lowvacuum and high surface pressure. In this way, a high level ofmechanical flatness can be provided for the encapsulated thermalmanagement structure.

[0060] The process of encapsulating the plate 10 in any one of thedescribed encapsulation materials maintains the thermal conductivity ofthe plate at substantially its pre-coating level. For example, whenthermalised pyrolytic graphite is used, the resulting thermal managementboard has an in-plane thermal conductivity of typically 1700W/mK at roomtemperature. It will be appreciated that for lower temperatures theconductivity is likely to be higher. This is advantageous. Anotheradvantage of the process of encapsulating the carbon is that theflatness of the thermal management board can be maintained at typicallyplus or minus 5 μm across a plate that is 100 mm by 100 mm, provided theoriginal material is suitably flat.

[0061] Using the encapsulation process described above it is possible toencapsulate, for example, a graphite plate 10 having a thickness of 200μm in a polyimide or epoxy resin or acrylic or polyurethane or polyesterlayer having a thickness in the range of 8-30 μm, preferably 15 μm. Thisresults in a thermal management board 13 having a total thickness in therange of 208-230 μm. Encapsulating the plate in this amount of materialresults in a board having a tensile strength that is significantlyhigher than that of the original carbon plate, thereby making the boardsufficiently strong for it to be handled easily. This is done with anegligible increase in volume and loss of thermal conductivity. This isunexpected and advantageous.

[0062] In many applications, thermal management devices are sandwichedbetween layers of printed circuit boards. Hence, it is advantageous tobe able to allow direct electrical interconnection between opposingsides of the device. In order to achieve this in the present case, priorto encapsulation, a matrix of holes is formed in the graphite plate 10by, for example, drilling. This is shown in FIG. 4. The holes 14 shouldeach have a diameter that is greater than the desired final diameter.Typically, the diameter of the holes 14 formed at this stage would be atleast 200 μm greater than the desired diameter. The holes 14 can ofcourse be formed in any desired layout. Polyimide or epoxy resin oracrylic or polyurethane or polyester 12 or any other suitable polymer isthen applied to the plate 10 in order to coat its surfaces and fill inthe holes 14, as shown in FIG. 5. If desired, the holes could in fact beinfilled with a mixture of the encapsulating material. For examplepolyimide, and glass spheres. This process can be carried out before thepure polyimide is used to encapsulate the surface of the plate. Thisimproves the uniformity of the coating thickness across the surface ofthem plate, by preventing the possibility of thinning occurring aroundthe edges of the initial holes. Once the plate is fully encapsulated, itis then processed as described above thereby to provide a rigid andhighly thermally conducting board.

[0063] In order to provide electrical connections through the board, thein-filled holes 14 are re-drilled to form holes 16 of a smallerdiameter, typically 100 μm or greater, as shown in FIG. 6. In this waypassages are formed through the board but the graphite 10 materialremains encapsulated in the polyimide or resin or acrylic orpolyurethane or polyester 12 or other suitable polymer and soelectrically insulated. Metal 18, such as aluminium is then deposited onboth sides of the board, as shown in FIG. 7, typically using thin filmaluminium processing techniques. Interconnection structures 20 aresubsequently etched using standard techniques on both sides of theboard, as shown in FIG. 8. In this way, a board 22 is produced havingmetalised holes through an encapsulated carbon plate, the metal of theholes being entirely insulated from the carbon 10.

[0064] The encapsulated thermal management board 13,22 can be used as aninterface to various assemblies. For example, it can be used for directthermal management of ceramic substrates such as alumina, beryllia andaluminium nitride, or metal substrates such as beryllium. This isachieved by applying, for example, a thin layer of liquid epoxy resin toone surface of the ceramic substrate, heating the substrate to 125° C.to polymerize the resin, and then positioning the substrate on thecarbon plate 10 or thermal management board 10,22. A high pressure, lowvacuum pressing at 180° C. is then applied to produce a bubble-freeinterface with a thickness of only a few microns. An alternative processis to coat the ceramic or metal substrate with a thin layer of liquidepoxy glue (typically a few microns thick), position it onto theanisotropic carbon plate or the thermal management board and attach itby allowing the epoxy to polymerise under pressing and low vacuum atroom temperature, in order to produce a bubble-free interface.

[0065] The thermal management board 13,22 can also be used as asubstrate for the custom design of thin film multi-layer circuits usingalternating layers of vacuum deposited aluminium and polyimide. Thealuminium 24 may be directly deposited onto the polyimide or resin oracrylic or polyurethane or polyester of the board 13,22 typically usingthin film aluminium techniques so that layers having thicknesses of 5 μmcan be deposited. FIG. 9 shows aluminium 24 deposited onto a layer ofepoxy resin 26 which is in turn deposited on one surface of plate 10.Because the coated surface of the plate 10 is flat, the resolution ofthe lithography used to deposit the aluminium 24 is good. This meansthat small features can be readily defined. Polyimide 28 is then appliedover the aluminum by spinning or screen-printing. Hence, the thicknessof the polyimide layer 28 can be, for example, as little as 8 μm. Usingstandard fabrication techniques, holes are then defined through thepolyimide 28 in appropriate places so that subsequent layers of metal 30that fill these holes can provide electrical contact to the aluminium24. Between the subsequent layers of metal 30 are, typically, layers ofpolyimide 28. Of course this processing could be done on opposing sidesof the plate 10 thereby to provide a double sided electrical componentwith an intrinsic thermal management capability.

[0066] The thin-film multi-layer circuits can also be fabricated onalternative substrates, for example aluminium, and subsequentlyseparated chemically. These circuits or other custom-designedmulti-layer circuits which may be fabricated on polyimide layers orepoxy resin based layers 31 can also be interfaced to the initialanisotropic carbon plate by applying, for example, a thin layer ofliquid epoxy glue 32 (typically a few microns thick) to the plate 10,placing the multi-layer circuit on that surface and allowing the epoxyto polymerise under pressing and low vacuum at room temperature, inorder to produce a bubble-free interface. A device that has beenfabricated by applying epoxy resin 32 to a carbon plate 10 is shown inFIG. 10. In this case the epoxy resin 32 acts both as a fixing agent tosecure the multi-layer circuit to the carbon plate 10 and additionallyas the material for encapsulating the carbon plate. In contrast, FIG. 11shows a multi-layer circuit that is interfaced using epoxy resin 32 witha surface of a carbon plate 10 that has been coated with, for example,polyimide 36.

[0067] In hybrid structures fabricated using any of the processesdescribed above changes in the temperature can cause variations in thelengths of the structural component layers. The changes in length forthe encapsulated board are different from those of the materials thatform the attached multi-layer hybrid structure. This affect degrades theoverall surface flatness, which can be a disadvantage in someapplications. It has been found, however, that optimum flatness can bemaintained, over a range of temperatures, typically 100C, by depositinga compensatory layer of encapsulating material on the opposite side ofthe thermal management board to that which carries the hybrid. Thiscompensatory layer should be of the same material and substantially thesame thickness as the layers of material that form the multi-layerhybrid structure. In this way, each side of the board has approximatelythe same co-efficient of thermal expansion and the overall flatness ofthe board does not vary substantially.

[0068] The compensatory layer can be mounted either by building upadditional layers of encapsulating material on the board until thedesired thickness is achieved or alternatively by gluing a cast of thematerial onto the surface of the board in a similar manner as previouslydescribed. As an example, FIG. 12 shows the structure of FIG. 9 ontowhich has been deposited a compensatory layer of polyimide 37 that has athickness that is roughly the same as the combined thickness of layers28 of FIG. 10, assuming in this case that the thickness of the hybridstructure is dominated by the layers of polyimide 28. As a furtherexample, FIG. 13 shows the structure of FIG. 11, onto which has beenglued a compensatory layer of polyimide 37, which has a thickness thatis roughly the same as the combined thickness of the layers 28 and 31 ofthe structure of FIG. 11. Again this assumes that the thickness of thehybrid structure is dominated by the layers 28 and 31.

[0069] To provide additional rigidity to the composite structure and/orto protect the edges against impact or delamination, the anisotropiccarbon plate can be inserted within a surrounding thin frame, which ispreferably made of material having the same co-efficient of thermalexpansion as the structure, for example, carbon fibre. In this way, asingle flat surface is provided that can be coated and attached to themulti-layer circuit in the room temperature process described above.

[0070] Hybrid devices that include, for example, multi-layer circuitsand a thermal management board can be made in various ways. In onetechnique, a plurality of such devices is fabricated from one largecarbon plate. FIG. 14 shows such a plate 38 on which are six processingsites 40. Each processing site is coated with, for example, polyimide,onto which the multi-layer circuits can be either directly deposited orfixed using epoxy glue. Once the processing at each site is completed,the plate 38 is cut up to form six discrete devices. The uncoated sidesof the carbon plate are then processed as previously described to ensurecomplete encapsulation of the carbon and formation of a thermalmanagement board. An advantage of this particular technique is thatproblems associated with the edges of the carbon plate 38 are avoided.

[0071] These procedures allow the thermal conductivity and low massproperty of the initial thermal management structure to be preservedafter interfacing with the custom-made multi-layer circuits.

[0072] As mentioned previously in some known applications whererelatively high thermal conductivity is required, beryllia substrateswith a layer of dielectric formed thereon have been used, and goldcontacts are deposited onto the dielectric. However, the hybridelectronic device that uses the thermal management device in which thepresent invention is embodied provides significantly higher thermalconductivity with a significant reduction in cost. Furthermore, becausethe materials involved are not hazardous, the fabrication of suchdevices is less problematic.

[0073] The process as described above allows the fabrication ofelectronic assemblies with high component densities constructed on ahigh thermal conductivity, low mass, graphite plate or core having thepossibility of customised electrical interconnections between itsopposing faces. This is achieved without the use of hazardous materials.

[0074] The thermal management board that uses thermalised pyrolyticgraphite has an optimal in-plane thermal conductivity typically of1550-185 mK at room temperature, whilst at the same time having a lowmass and easy to handle structure. In addition, the substrates can bereadily used as interfaces between other circuits. Furthermore, anygeometry of the carbon can be used prior to encapsulation SD that thethermal management device can be custom made for each particularapplication. This is advantageous because it means that the applicationof the process is not significantly limited.

[0075] In the use of thermal management structures for coolingelectrical systems, thermal grease is often used as an interface. It isenvisaged that a thermal management device in which the invention isembodied could be used in place of the grease, it being appreciated thatthe device used in such an application should be relatively thin.

[0076] The skilled person will appreciate that variations of thedisclosed arrangements are possible without departing from theinvention. Accordingly, the above description of several embodiments ismade by way of example and not for the purposes of limitation. Inaddition, it will be clear to the skilled person that minormodifications can be made without significant changes to the conceptsdescribed above.

1. A thermal management device comprising anisotropic carbonencapsulated in an encapsulating material that is applied directly tothe anistropic carbon and improves the rigidity of the carbon,preferably wherein the encapsulating material is polyimide or epoxyresin or acrylic or polyurethane or polyester or any other suitablepolymer.
 2. A thermal management device as claimed in claim 1, whereinthe anisotropic carbon has mosaic or fill ordering.
 3. A thermalmanagement device as claimed in claim 1 or 2, wherein the anisotropiccarbon is thermalised pyrolytic graphite.
 4. A thermal management deviceas claimed in claim 3, wherein the thermalised pyrolytic graphite has anin plane thermal conductivity in the range of 1550-1850W/mK at roomtemperature.
 5. A thermal management device as claimed in claim 3 or 4,wherein the thermalised pyrolitic graphite has a low value of tensilestrength in the orthogonal direction.
 6. A thermal management device asclaimed in claim 1, wherein the anisotropic carbon is pyrolyticgraphite.
 7. A thermal management device as claimed in claim 6, whereinthe pyrolytic graphite is in an “as deposited” or partially orderedform.
 8. A thermal management device as claimed in claim 6 or 7, whereinthe conductivity of the pyrolytic graphite is in the range of300-420W/mK in one plane.
 9. A thermal management device as claimed inany one of claims 6 to 9, wherein the tensile strength of the carbon is1.5 Ksi in the orthogonal plane.
 10. A thermal management device asclaimed in any one of the preceding claims, wherein the anisotropiccarbon is a plate.
 11. A thermal management device as claimed in claim10, wherein the carbon plate has a thickness in the range 100-500 μm,preferably 200-250 μm or 250, 300 μm or 300-350 μm or 350-400 μm or400-450 μm or 400-500 μm.
 12. A thermal management device as claimed inany one of the preceding claims, wherein the material encapsulating thecarbon has a low thermal expansion coefficient and high degradationtemperature.
 13. A thermal management device as claimed in any one ofthe preceding claims wherein the encapsulating layer has a thickness inthe range from a few microns to many tens of microns.
 14. A thermalmanagement device as claimed in any one of the preceding claims whereinmultiple layers of encapsulating material are deposited on the carbon inorder to build up a desired thickness.
 15. A thermal management deviceas claimed in any one of the preceding claims, wherein a matrix of fineholes, preferably each hole having a diameter of 200 μm diameter, isformed through the carbon.
 16. A thermal management device as claimed inclaim 15, wherein the holes are filled during encapsulation of theplate.
 17. An electrical system comprising a thermal management deviceas claimed in any of the preceding claims, on a surface of whichelectrical contacts and/or devices are provided.
 18. An electricalsystem as claimed in claim 17, wherein the devices are depositeddirectly on the surface of the thermal management device or are gluedusing, for example, a thin layer of liquid glue.
 19. An electricalsystem as claimed in claim 17 or 18, wherein the devices areencapsulated in polyimide or epoxy resin or acrylic or polyurethane orpolyester or any other suitable polymer.
 20. An electrical system asclaimed in any one of claims 17 to 19, wherein a plurality of layers ofelectrical components are provided.
 21. An electrical system as claimedin claim 20, wherein each layer of electrical components is spaced apartby layers of encapsulating material, preferably polyimide.
 22. Anelectrical system as claimed in any one of claims 17 to 21, wherein theelectrical contacts are made of thin film metal, preferably aluminium.23. A method of fabricating a thermal management device comprising:applying a coat of encapsulating material, preferably polyimide or epoxyresin or acrylic or polyurethane or polyester or any other suitablepolymer directly to a clean carbon surface, repeating the foregoingsteps until the carbon is encapsulated.
 24. A method as claimed in claim23 that further involves curing the encapsulating material.
 25. A methodas claimed in claim 23 or claim 24, wherein the step of applyinginvolves brushing, rolling, dipping, spraying, spinning, stamping orscreen-printing.
 26. A method as claimed in claim 25, wherein forpolyimide, which consists of a single-component, the step of applyingthe coating involves brushing the polyimide or applying it using aroller.
 27. A method as claimed in claim 23 or claim 24, wherein forsolid phase application a cast is used.
 28. A method as claimed in claim27, wherein the carbon and cast are pressed together within a vacuum andat high temperature.
 29. A method as claimed in any one of claim 23 to28, wherein multiple layers of encapsulating material are applied untila desired thickness is reached.
 30. A method as claimed in any one ofclaims 23 to 29 including cleaning a surface of the carbon thereby toproduce said clean carbon surface.
 31. A method as claimed in claim 30,wherein the step of cleaning involves using pumice powder under water toremove loose materials, followed by drying.
 32. A method as claimed inclaim 31, wherein the step of drying involves drying the carbon bybaking the carbon surface to remove moisture
 33. A method as claimed inclaim 32, wherein the step of drying involves baking the carbon at 100Cfor one hour.
 34. A method as claimed in any one of claims 30 to 33,wherein the step of cleaning involves degreasing the carbon, preferablyby rinsing it with acetone.
 35. A method as claimed in any one of claims23 to 34, wherein when polyimide is used, the step of curing involvesheating the carbon to substantially 150C for, preferably, 1 hour andsubsequently temperature cycling the board to 150C for 30 minutes, 250Cfor 30 minutes and finally 300C for 30 minutes.
 36. A method as claimedin any one of claims 23 to 35, further comprising drilling the carbonwith at least one hole prior to application of the encapsulatingmaterial.
 37. A method as claimed in claim 36, wherein the at least onehole is infilled with encapsulating material.
 38. A method as claimed inclaim 37, wherein tile holes are infilled with encapsulating materialthat is mixed with glass fibre spheres, each sphere typically having adiameter of 30 μm.
 39. A method as claimed in any one of claims 36 to38, wherein the infilled holes are drilled thereby to provide throughpassages that are electrically isolated from the carbon.
 40. A method asclaimed in claim 39, wherein a layer of a conducting material is appliedto the at least one through passage to produce electrical connections,thereby to enable electrical connections through the carbon.
 41. Amethod as claimed in claim 40, wherein the conducting material is ametal, preferably thin film aluminium.
 42. A method as claimed claim 36,wherein edges that define the at least one hole are coated with theencapsulating material in such a way as to maintain a passage throughthe carbon.
 43. A method as claimed in any one of claims 23 to 42,further involving forming a matrix of fine holes through the plate. 44.A method of fabricating an electrical component comprising the method ofas defined in any one of claim 23 to 43, additionally comprising thesteps of forming electrical contacts on at least one surface of thecarbon and/or depositing electrical devices thereon.
 45. A method asclaimed in claim 44, wherein the step of depositing may involvefabricating the devices directly on the surface or forming the devicesor a thin film multi-layer circuit containing the devices separatelyfrom the carbon surface and fixing them to that surface.
 46. A method asclaimed in claim 45, wherein the step of fixing involves applying epoxyglue to the devices or the circuit or the carbon surface and pressingthe devices or circuit and the surface together at room temperature andat low vacuum.
 47. A method as claimed in any one of claims 44 to 46,wherein the electrical contacts are applied using thin film processingtechniques, preferably using aluminium.